Field of the Application
The present invention generally relates to a control apparatus for an internal combustion engine. In particular, it relates to a control apparatus for an internal combustion engine provided with a turbocharger.
Background Art
Japanese Patent Laid-Open No. 8-177569 (Patent Literature 1) discloses a control apparatus for an internal combustion engine that switches an operation mode between a stoichiometric operation in which fuel combustion occurs at an air-fuel ratio close to a theoretical air-fuel ratio and a lean operation in which fuel combustion occurs at an air-fuel ratio leaner than the theoretical air-fuel ratio, the controller being designed to change a reference value for switching the operation mode between the stoichiometric operation and the lean operation depending on various conditions such as the inclination of the road surface on which the vehicle is running and environmental conditions. During gentle slope climbing or low-altitude driving, this controller raises the reference value to achieve the maximum torque in the lean operation and can thereby improve fuel consumption. During steep slope climbing or high-altitude driving, this controller lowers the reference value in preparation for an increase of the torque and can thereby improve drivability.
Switching between the stoichiometric operation and the lean operation is performed based on an operation region map that defines a region in which the stoichiometric operation occurs (referred to as a stoichiometric region hereinafter) and a region in which the lean operation occurs (referred to as a lean region hereinafter). The operation region map is set in advance by adaptation or the like. In the operation region map, the boundary between the stoichiometric region and the lean region is set based on the maximum value of the engine torque output when the internal combustion engine is in the lean operation with the throttle fully opened (referred to as a maximum torque value hereinafter).
In general, the maximum torque value is controlled by the amount of air taken in by the internal combustion engine when the throttle is fully opened (referred to as a WOT air amount hereinafter). However, if the WOT air amount decreases because of deposit formation on the throttle valve, the intake valve or the like or the back pressure increases because of plugging of an exhaust catalyst, the maximum torque value decreases. In this regard, if the internal combustion engine is provided with a turbocharger, the WOT air amount may further decrease because of deposit formation on the compressor, and the turbocharging efficiency can decrease because of various factors. Thus, with the internal combustion engine provided with a turbocharger, the maximum torque value is more likely to decrease than a non-turbocharged internal combustion engine.
If the maximum torque value decreases, the stoichiometric operation is needed to achieve the torque in a part of the lean region in the preset operation region map. However, since the boundary is fixed, an operation region occurs between the boundary and the reduced maximum torque value in which the engine torque does not increase even if the accelerator pedal is further pressed down. If such an operation region occurs, the acceleration demanded by the driver cannot be achieved, and the drivability can deteriorate.